Cast and thermoplastic polyurethane compositions based on the reaction of polyisocyanates with polymeric diols are well known for use as elastomers, adhesives, sealants, elastomeric surface coatings, and coatings for metals and plastics. Kuraray markets a partially hydrogenated polyisoprene diol which is described in product brochures as useful in making polyurethanes when reacted with isocyanates and various chain extenders. The diol has a number average molecular weight of 3800, a broad molecular weight distribution and a hydroxyl content of about 2.2 terminal hydroxyl groups per molecule. Typically the hydrogenation of this product is around 80%.
The isocyanates described by Kuraray include MDI, IPDI, and TDI. The chain extenders described by Kuraray include 1,4-butane diol, 2-ethyl-1,3-hexane diol, 3-methyl-1,5-pentane diol and 1,9-nonane diol. The polyurethanes have properties consistent with addition of a 3800 molecular weight partially hydrogenated polyisoprene to the polyurethane structure, including good hydrolysis resistance.
The average functionality of the Kuraray materials being above 2 makes these products unsuitable for thermoplastic polyurethane application. Thermoplastic polyurethanes (TPU) allow the production of elastomeric materials by thermoplastic processing techniques. TPU's may not thermally degrade when subjected to temperatures and pressures necessary for melt processing. Therefore the TPU macromolecules have to be linear and not substantially branched macromolecules which cannot be repeatedly thermoformed. Only bifunctional isocyanates, chain extenders and long chain diols can be used to make thermoplastic polyurethanes.
It is an object of the present invention to provide thermoplastic polyurethane compositions having improved resistance to hydrolytic degradation caused by exposure to water at elevated temperatures for short times.